Malfunction determining apparatus of an exhaust gas recirculation system

ABSTRACT

A malfunction determining apparatus of an exhaust gas recirculation system can prevent an erroneous determination that an exhaust gas recirculating valve is normal when the valve has a cause of malfunctioning. The exhaust gas recirculating valve is provided between an exhaust passage and an intake passage of an internal combustion engine of a vehicle. An operation determining part determines whether the exhaust gas recirculating valve has been operated at least once. A malfunction determining part performs a determination as to whether the exhaust gas recirculation system is malfunctioning. The determination is performed after the exhaust gas recirculating valve has been operated at least once.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a malfunction determining apparatus ofan exhaust gas recirculation system and, more particularly, to amalfunction determining apparatus of an exhaust gas recirculation systemwhich detects a malfunction of an EGR valve of the exhaust gas recyclingsystem.

2. Description of the Related Art

An exhaust gas recirculation system (hereinafter referred to as an EGR)is known as means for purifying exhaust gas of an internal combustionengine (hereinafter referred to as an engine). The EGR reduces NOx byrecirculating a part of exhaust gas to an intake line.

In an engine having the EGR, since the amount of smoke is increased asthe recirculation rate (a ratio of an exhaust gas to a fresh gas) isincreased, the amount of recirculation of exhaust gas cannot beincreased beyond a certain limit. Additionally, since the amount ofsmoke tends to be increased as the load of the engine is increased, atarget exhaust gas recirculation rate is varied in response to operatingconditions of the engine by providing an exhaust gas recirculating valve(EGR valve) in an exhaust gas recirculation passage. An open/closeoperation of the EGR valve is controlled in response to the targetexhaust gas recirculation rate so that the maximum recirculation ofexhaust gas can be achieved within a range in which an excessive amountof smoke is not generated.

In a case where the above-mentioned EGR valve is fixed in an open statedue to malfunctioning, exhaust gas is continuously recirculated to anintake line. This may increase the amount of smoke and cause an enginestall. Accordingly, a malfunction determining apparatus has beensuggested to detect a malfunction of the EGR.

A malfunction determining apparatus of the above-mentioned type isdisclosed in Japanese Laid-Open Patent Application No. 4-103865. Themalfunction determining apparatus of the EGR disclosed in theabove-mentioned publication detects an actual degree of opening of theEGR valve (actual valve opening degree), and sets a target openingdegree of the valve in response to an operating condition of the engine.It is determined that a malfunction occurs in the EGR when a differencebetween the actual valve opening degree and the target opening degree isgreater than a predetermined value.

The above-mentioned EGR valve is operated by a pressure supplied by avacuum switching valve (duty-VSV) which is controlled by an EGR computerto switch the pressure between a negative pressure in an intake pipe andan atmospheric pressure with a variable duty ratio. Thus, if atmosphericair passage of the duty-VSV is clogged, the atmospheric air cannot beintroduced into the EGR valve. Accordingly, since a negative pressure inthe EGR valve cannot be released, the EGR valve is maintained in an openstate which results in a close malfunction state.

However, when the EGR valve is not operated for a certain time period,air is gradually introduced through the atmospheric air passage and thusthe EGR valve is closed. In a state where the EGR valve is not operated,the target valve opening degree is set to zero. Additionally, when theEGR valve is closed due to introduction of air through the atmosphericair passage as mentioned above, the actual valve opening degree is alsozero.

Accordingly, if a determination of malfunctioning is performed in such astate, the difference between the actual valve opening degree and thetarget valve opening degree is less than a predetermined value. Thus,there is a problem in that an erroneous determination is made that theEGR valve is normal even though the EGR valve has a cause ofmalfunctioning due to the atmospheric air passage being clogged.

SUMMARY OF THE INVENTION

It is a general object of the present invention to provide an improvedand useful malfunction determining apparatus of an exhaust gasrecirculation system in which the above-mentioned problems areeliminated.

A more specific object of the present invention is to provide amalfunction determining apparatus of an exhaust gas recirculation systemwhich can prevent an erroneous determination that the EGR valve isnormal even though the EGR valve is actually malfunctioning.

In order to achieve the above-mentioned objects, there is providedaccording to the present invention a malfunction determining apparatusof an exhaust gas recirculation system having an exhaust gasrecirculating valve provided between an exhaust passage and an intakepassage of an internal combustion engine of a vehicle, the malfunctiondetermining apparatus comprising:

operation determining means for determining whether the exhaust gasrecirculating valve has been operated at least once; and

malfunction determining means for performing a determination as towhether the exhaust gas recirculation system is malfunctioning, thedetermination of malfunctioning being performed after the exhaust gasrecirculating valve has been operated at least once.

According to the above-mentioned invention, the determination ofmalfunctioning is performed after the exhaust gas recirculating valvehas been operated at least once. When the exhaust gas recirculatingvalve is operated at least once and if the exhaust gas recirculatingvalve has a cause of malfunctioning, the exhaust gas recirculating valvemaintains an open state after operation. Accordingly, an accuratedetermination of malfunctioning can be made by performing themalfunction determining process when a malfunction actually occurs inthe exhaust gas recirculating valve.

The malfunction determining apparatus according to the present inventionmay further comprises a start condition determining part whichdetermines an appropriate condition to start a malfunction determiningprocess to make the determination of malfunctioning based on operatingconditions of the internal combustion engine and the vehicle.Accordingly, the malfunction determining process can be started in acondition in which an accurate determination of malfunctioning can beperformed.

The start condition determining part may determine whether the internalcombustion engine has been operated for a predetermined period so as tostart the malfunction determining process after the internal combustionengine has been operated for the predetermined period.

Additionally, the start condition determining part may determine whetherthe internal combustion engine is in an idle operation and the vehicleis in a stopped state so as to start the malfunction determining processwhen the internal combustion engine is in an idle operation and thevehicle is in the stopped state.

Further, the start condition determining part may determine whether atemperature of the cooling water of the internal combustion engine isequal to or greater than a predetermined temperature so as to start themalfunction determining process when the temperature of the coolingwater of the internal combustion engine is equal to or greater than thepredetermined temperature.

Additionally, the start condition determining part may determine whetherthe exhaust gas recirculating valve is controlled to be in a closedstate so as to start the malfunction determining process when theexhaust gas recirculating valve is controlled to be in the closed state.

Other objects, features and advantages of the present invention willbecome more apparent from the following detailed description when readin conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a structure of an exhaust gas recirculationsystem provided with a malfunction determining apparatus according to anembodiment of the present invention;

FIG. 2 is a block diagram showing a structure of the malfunctiondetermining apparatus according to the embodiment of the presentinvention;

FIG. 3 is a flowchart showing a start condition determining processwhich is performed by the ECU provided in the malfunction determiningapparatus according to the embodiment of the present invention;

FIG. 4 is a flowchart showing the malfunction determining process whichis performed by the ECU provided in the malfunction determiningapparatus according to the embodiment of the present invention;

FIG. 5 is a timing chart for explaining a conventional start conditiondetermining process;

FIG. 6 is a timing chart for explaining the start condition determiningprocess according to the embodiment;

FIG. 7 is a timing chart for explaining the malfunction determiningprocess according to the present invention;

FIG. 8 is an illustration for explaining a reason for using a bluntedamount of a target lift in the malfunction determining process accordingto the present invention;

FIG. 9 is an illustration for explaining a reason for using both anamount of target lift and the blunted amount of the target lift;

FIG. 10 is an illustration for eliminating a problem which may occur ina high-loaded condition in the malfunction determining process accordingto the present invention; and

FIG. 11 is a flowchart of an EGR control process.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A description will now be given, with reference to the drawings, of theinvention.

FIG. 1 is an illustration of a structure of an exhaust gas recirculationsystem 1 (hereinafter referred to as EGR) provided with a malfunctiondetermining apparatus according to an embodiment of the presentinvention. In the present embodiment, the EGR 1 is provided to aninternal combustion engine 2 (hereinafter referred to as engine).

The EGR 1 comprises an exhaust gas recirculating passage 3, an exhaustgas recirculating valve 4 (hereinafter referred to as EGR valve), avacuum switching valve 5 (hereinafter referred to as VSV), a vacuumcontrol valve 6 (hereinafter referred to as VCV), a vacuum tank 7 and anelectronic control unit 8 (hereinafter referred to as ECU).

An intake passage 9 is connected to an intake port of the engine 2. Anexhaust passage 10 is connected to an exhaust port of the engine.Additionally, a throttle valve 11, which is operated in accordance withthe amount of depression of an acceleration pedal, is provided in theintake passage 9. A three-way catalytic converter 12 is provided in theexhaust passage 10 to decrease the amount of harmful components such asHC (hydrocarbon), CO (carbon monoxide) and NOx (nitrogen oxides) in theexhaust gas.

The exhaust gas recirculating passage 3 is provided between the intakepassage 9 and the exhaust passage 10 as a bypass passage. An end of theexhaust gas recirculating passage 3 is connected to the downstream sideof the throttle valve 11 of the intake passage 9. An opposite end of theexhaust gas recirculating passage 3 is connected to the upstream side ofthe three-way catalytic converter 12 of the exhaust passage 3. Thus, theintake passage 10 is connected to the exhaust passage 9 by the exhaustgas recirculating passage 3.

The EGR valve 4 is provided in the middle of the exhaust gasrecirculating passage 3. The interior of the EGR valve 4 is divided intoan atmospheric pressure chamber 15 and a diaphragm chamber 16 by adiaphragm 14 provided in a casing 13. A valve shaft 17 is connected tothe diaphragm 14. A valve body 18 is provided to a lower end of thevalve shaft 18 so as to open and close the exhaust gas recirculatingpassage 3. The above-mentioned atmospheric chamber 15 is open to theatmosphere. The diaphragm chamber 16 is connected to the VSV 5 vianegative pressure introducing pipes 21 and 20 and the vacuum tank 7.

The diaphragm 14 is urged downwardly in the figure by a coil spring 30provided in the diaphragm chamber 16. The diaphragm 14 moves upwardlywhen a negative pressure is introduced into the diaphragm chamber 16from the VSV 5. Accordingly, the valve body 18 is moved upwardly by theupward movement of the valve shaft 17 connected to the diaphragm 14which causes the EGR valve to open. The degree of opening of the EGRvalve 4 is controllable by the negative pressure introduced into thediaphragm chamber 16.

As mentioned above, the intake passage 9 and the exhaust passage 10 areconnected to each other when the EGR valve 4 is open. Thus, exhaust gasflowing in the exhaust passage 10 is returned to the intake passage 9.Additionally, the amount of exhaust gas returned from the exhaustpassage 10 to the intake passage 9 can be controlled by controlling thedegree of opening of the EGR valve 4.

The valve shaft 17 extends above the diaphragm 14. The extended portionof the valve shaft 17 is connected to a lift sensor 19 provided in anupper portion of the casing 13. The lift sensor 19 is a position meterusing a resistor. The lift sensor functions as means for detecting anactual degree of opening (an actual state of valve operation) of the EGRvalve 4.

Specifically, the lift sensor 19 comprises a contact brush connected tothe valve shaft 17 and a resistor placed in contact with the contactbrush. The degree of opening of the valve body 18 is detected bymeasuring a voltage which varies in response to a position of thecontact brush where the contact brush is in contact with the resistor.The voltage representing a degree of opening of the EGR valve 4 which isdetected by the lift sensor 19 and is supplied to the ECU 8.

The VSV 5 has a function to control the negative pressure to beintroduced into the EGR valve 4. The VSV 5 comprises an atmospheric airport 22, a negative pressure introducing port 23, an output port 24, anspool 25 and a coil 26.

The atmospheric air port 22 is connected to the upstream side of thethrottle valve 11 of the intake passage 9 via an atmospheric pressureintroducing pipe 27 so as to introduce atmospheric air into the VSV 5.The negative pressure introducing port 23 is connected to the downstreamside of the throttle valve 11 of the intake passage 9 via negativepressure introducing pipes 28 and 29 and the VCV 6 so as to introduce anegative pressure into the VSV 5. The output port 24 is connected to thediaphragm chamber 16 of the EGR valve 4 via the negative pressureintroducing pipes 20 and 21 and the vacuum tank 7.

Additionally, the coil 26 is electrically connected to the ECU 8. Thecoil 26 is excited by a drive signal (duty signal) supplied by the ECU 8so that the spool 25 is moved leftwardly and rightwardly in the figure.The output port 24 is selectively connected to the atmospheric air port22 or the negative pressure introducing port 23 by the movement of thespool 25. Since the movement of the spool 25 can be controlled by a dutyratio of the duty signal supplied by the ECU 8, the level of thenegative pressure introduced into the EGR valve 4 can be controlled.That is, a degree of opening of the EGR valve 4 is controlled bycontrolling an operation of the VSV 5 by the ECU 8.

The VCV 6 has an input port 31 and an output port 32. The input port 31id connected to the intake passage 9 via the negative pressure pipe 28.The output port 32 is connected to the negative pressure introducingport 23 of the VSV 5 via the negative pressure introducing pipe 29. TheVCV 6 includes a diaphragm 33 and a valve body 34 therein. The valvebody 34 opens and closes by movement of the diaphragm 33. The diaphragmis moved in response to a level of the negative pressure of the intakeline which is introduced into the input port 31. The valve body 34 isopened and closed in response to the movement of the diaphragm 33, andthereby the negative pressure output from the output port 32 isadjusted. Accordingly, the negative pressure at the output port 32 ofthe VCV 6 is maintained at a constant level irrespective of fluctuationof the negative pressure in the intake line.

An input port 35 of the vacuum tank 7 is connected to the output port 24of the VSV 5 via the negative pressure introducing pipe 20. An outputport of the vacuum tank 7 is connected to the diaphragm chamber 16 ofthe EGR valve 4 via the negative pressure introducing pipe 21. Thevacuum tank 7 has a relatively large volume so that the vacuum tank 7removes pulsations when the negative pressure output from the VSV5includes such pulsations.

It should be noted that, although not shown in FIG. 1, an ignitionswitch 37 which detects and enables starting of the engine 2, a watertemperature sensor 38 which detects a cooling water temperature THW ofthe engine 2, a vehicle speed sensor 39 which detects a speed SPD of thevehicle and a throttle switch 54 which detects a completely closed stateof the throttle valve 11 are connected to the ECU 8 in addition to thelift sensor 19.

A description will now be given, with reference to FIG. 2, of theabove-mentioned ECU 8. The ECU 8 comprises a logic operation circuitincluding a central processing unit (CPU) 40, a read only memory (ROM)41 which stores predetermined control programs and maps, a random accessmemory (RAM) 42 which temporarily stores calculation results of the CPU40, a back-up RAM 43 which stores predetermined data, a clock generator(CLOCK) 44 which generates a predetermined clock signal, and a bus 47which connects an input port 45 and an output port 46 to each component.

The above-mentioned lift sensor 19, the ignition switch 37, the watertemperature sensor 38, the vehicle speed sensor 39 and the throttleswitch 54 are connected to the input port 45 via buffers 48 and 49, amultiplexer 50, an A/D converter 51 and a waveform shaping circuit 52.The CPU 40 reads detection signals of each sensor 19 and 37-39 which areinput via the input port 54. Additionally the VSV 5 is connected to theoutput port 46 via a drive circuit 53. In this structure, the CPU 40controls VSV 5 based on the input from each of the sensors 19 and 37-39,and performs a malfunction determining process as a part of the presentinvention described below.

A description will now be given, with reference to FIGS. 3 to 10, of themalfunction determining process performed by the EGR 1. The malfunctiondetermining process related to the present embodiment is performed todetermine an occurrence of a malfunction (closing malfunction) in whichthe valve body 18 of the EGR 1 stays in an open position. FIG. 3 is aflowchart showing a start condition determining process for determininga start condition which provides a start time of the malfunctiondetermining process. FIG. 4 is a flowchart showing a malfunctiondetermining process which is performed when the start time of themalfunction determining process is determined by the start conditiondetermining process.

A description will given below of the start time determining process ofthe malfunction determining process.

Prior to the description of the start condition determining processshown in FIG. 3, a description will be given first, with reference toFIGS. 5 and 6, of the principle of the start condition determiningprocess. FIG. 5 is a timing chart of the EGR 1 shown in FIG. 1 when theEGR 1 is operated in accordance with a conventional start conditiondetermining process. FIG. 6 is a timing chart when the EGR 1 is operatedin accordance with the start condition determining process according tothe present embodiment.

Reference is made to the conventional start condition determiningprocess shown in FIG. 5. Conventionally, the start time of themalfunction determining process is set irrespective of the presence ofan operational history of the EGR valve 4 as mentioned above.Specifically, the malfunction determining process is started immediatelyat a time (time T1) when a predetermined time period t1 elapses afterthe engine is started.

It is assumed that the vehicle starts to move at a time T2, and,consequently, an operation of the EGR 1 is started at a time T3 (T3>T1)as shown in FIG. 5-(A). In such a case, there is a possibility that themalfunction determining process is completed before the EGR valve 4 isoperated if a long time is taken for the vehicle to start to move.

Reference is now made to the actual valve opening degree of the EGRvalve 4 indicated by a solid line and a malfunction determining valuecalculated by the ECU 8 indicated by a dashed line in FIG. 5-(C). In theconventional malfunction determining process, the target valve openingdegree which is calculated by the ECU 8 corresponds to the malfunctiondetermining value. As shown in FIG. 5-(C), the malfunction determiningvalue calculated by the ECU 8 is set to a small value since the engine 2is in an idle state at the time T1 when the malfunction determiningprocess is started.

If it is assumed that the atmospheric air port 22 of the VSV 5 isclogged by ice, atmospheric air cannot be introduced into the diaphragmchamber 16 of the EGR valve 4. This results in a closing malfunctionstate in which the EGR valve 4 cannot be closed. However, even thoughthe EGR valve 4 has the above-mentioned malfunctioning factor, the EGRvalve 4 may be gradually closed by air introduced through theatmospheric air port 22 when the EGR 4 is not operated for aconsiderable time period.

Accordingly, if the malfunction determining process is performed beforethe EGR valve is operated, the difference (indicated by dL1 in thefigure) between the malfunction determining value and the actual valveopening degree is reduced to less than a predetermined value. This leadsto the erroneous determination by the ECU 8 that the operation of theEGR valve 4 is normal.

Specifically, as shown in FIG. 5-(B), when the malfunction determiningprocess is started at the time T1, a malfunction determining opportunitycounter ECDEGOF starts to operate so that the malfunction determiningprocess is started each time the malfunction determining opportunitycounter ECDEGOF is incremented. However, before the EGR valve 4 startsto operate as mentioned above, the difference dL1 between themalfunction determining value and the actual valve opening degree isless than a predetermined value and, thus, the malfunction determinationis not performed. Accordingly, a malfunction determining counter CDEGOF,which is incremented each time the malfunction determination isperformed, remains at zero as shown in FIG. 5-(D). Thus the ECU 8erroneously determines that the operation of the EGR valve 4 is normal.

When the operation of the EGR 1 is started at the time T3 under thiscondition, the EGR valve 4 stays in an open state as indicated by asingle dashed chain line of FIG. 5-(C) since the atmospheric air port 22is clogged. This causes malfunctioning of the EGR 4. In this state, thedifference between the malfunction determining value and the actualvalve opening degree becomes large as indicated by dL2.

Accordingly, the ECU 8 perform an EGR control process based on thedetermination that the operation of the EGR 4 is normal in accordancewith the result of the malfunction determining process which wasperformed before the EGR valve 4 is operated.

On the other hand, in the start condition determining process accordingto the present embodiment, the malfunction determination is performedafter the EGR valve 4 is operated. A description will now be given, withreference to FIG. 6, of the start condition determining processaccording to the present invention.

In the start condition determining process according to the presentembodiment, an EGRON history flag XJEGON is provided which is set whenthe EGR valve 4 is operated to open (EGR ON) as shown in FIG. 6-(C). Theexecution of the malfunction determining process is prohibited until theEGRON history flag XJEGON is set, that is, until the time T3 is reached.Thus, the malfunction determining process is performed after the EGRONhistory flag XJEGON is set, that is, after the time T3 is reached. Inthe example shown in FIG. 6, the malfunction determining process isperformed at a time T4.

By executing the malfunction determining process after the EGR valve 4is operated, a reliable detection of malfunction can be made even whenthe VSV 5 has a cause of malfunction as in the case where theatmospheric air port 22 is frozen. That is, the EGR valve 4 ismaintained in the closed state after the time T3 when the EGR valve 4 isoperated since the atmospheric air is not introduced from the VSV 5.Thus, determination of an occurrence of malfunction is made under thecondition in which the EGR valve 4 is malfunctioning by executing themalfunction determining process after the EGR valve 4 was operated, thatis, after the EGRON history flag XJEGON was set. Thus, the ECU 8 canpositively determine the occurrence of malfunction in the EGR valve 4.

Specifically, as shown in FIG. 6-(B), when the malfunction determiningprocess is started at the time T4, the operation of the malfunctiondetermining opportunity flag ECDEGOF is started. The malfunctiondetermining process is executed each time the malfunction determiningopportunity counter ECDEGON is incremented. However, after the EGR valve4 has started, the difference dL2 between the malfunction determiningvalue and the actual valve opening degree is greater than apredetermined value. Thus, the ECU 8 performs the malfunctiondetermining process, and thereby the malfunction determining counterCDEGOF is incremented. In the present embodiment, the finaldetermination of the occurrence of malfunction (final malfunctiondetermining process) is performed when the malfunction determiningcounter CDEGOF becomes equal to 10 (CDEGOF=10).

It should be noted that the malfunction determining opportunity counterECDEGOF is provided and the final malfunction determination is performedwhen a malfunction is detected a plurality of consecutive times (totimes in this embodiment) because it is possible that an erroneousdetermination is made due to an external disturbance when the finalmalfunction determination is made by performing the malfunctiondetermination only once.

Additionally, in the present embodiment, an amount of lift ELIFTD1 atthe time T3 when an operation of the EGR 1 is started is used as themalfunction determining value. The reason for this will be describedlater.

A description will now be given, with reference to the flowchart of FIG.3, of the start condition determining process of the malfunctiondetermining process which is performed based on the above-mentionedprinciple.

When the malfunction determining process is started, first it isdetermined, in step 10, whether or not an EGR start flag XAEGR is set.The EGR start flag XAEGR is a flag which is set when an operation of theEGR 1 is started by the ECU 8. The EGR start flag XAEGR is set in anexhaust gas recirculation control process (hereinafter referred to asEGR control process) which is separately performed from the malfunctiondetermining process shown in FIG. 11.

If an affirmative determination is made in step 10, the process proceedsto step 12 where the EGRON history flag XJEGON is set (XJEGON=ON).Accordingly, it can be determined by checking a state of the EGRONhistory flag XJEGON whether the EGR 1 was operated at least one time andconsequently the EGR valve 4 was operated at least one time. That is,the operation history of the EGR valve 4 can be known by checking thestate of the EGRON history flag XJEGON.

The EGRON history flag XJEGON is cleared when the ignition switch 37 isturned off. That is, if the EGRON history flag XJEGON is set once, theset state (XJEGON=0) of the flag XJEGON (XJEGON=ON) is maintained untilthe engine 2 is stopped. Accordingly, when an operation of the EGR 1 wasstarted and, thereafter, the operation of the EGR 1 is stopped due to anoperating condition of the engine 2, the EGRON history flag XJEGON ismaintained in the set state.

When the process of the step 12 is completed, or when a negativedetermination is made in step 10, the process proceeds to step 14. Instep 14, above-mentioned malfunction determining opportunity counter isincremented by one count (refer to FIG. 6(B)).

In the successive steps 16-24, processes for determining whether or notthe condition of the engine 2 is appropriate for determining anoccurrence of malfunction. In step 16, it is determined whether or notthe predetermined time period t2 has elapsed, Means for measuring thepredetermined time period t2 are provided by using the clock 44 in theECU 8 (refer to FIG. 2) and providing a counter which is started whenthe ignition switch 37 is turned on.

If a negative determination is made in step 16, the engine 2 is in astate immediately after start where the predetermined time period hasnot elapsed after the starting. In the state immediately after start, itis possible that the engine 2 is in an unstable condition, and thus itis not appropriate to perform the malfunction determining process. Thus,if the negative determination is made in step 16, the process proceedsto step 26 so as to clear the malfunction determining opportunitycounter ECDEGOF, and then the process is returned to step 10.

On the other hand, if an affirmative determination is made in step 16,the process proceeds to step 18 where it is determined whether or notthe engine 2 is in an idle state and also if movement of the vehicle isstopped. It can be detected by an output signal of the throttle switch54 whether or not the engine 2 is in an idle state. It can be detectedby an output signal of the vehicle speed sensor 39 whether or not thevehicle is in a stopped state.

If a negative determination is made in step 18, this means that thevehicle is in a moving state. In the moving state of the vehicle, it ispossible that operating conditions of the engine 2 change. Thus, it isnot appropriate to determine occurrence of a malfunction. Thus, if thenegative determination is made in step 18, the process proceeds to step26 so as to clear the malfunction determining opportunity counterECDEGOF, and then the process is returned to step 10.

On the other hand, if an affirmative determination is made in step 18,the process proceeds to step 20 where it is determined whether or notthe cooling water temperature THW is greater than a predeterminedtemperature k. The cooling water temperature THW can be detected by anoutput signal of the water temperature sensor 38.

If a negative determination is made in step 20, the engine 2 is notsufficiently warmed-up. In such a cool state, it is possible that theoperation of the engine 2 is not stable. Thus, it is not appropriate toperform a determination of occurrence of malfunction. Accordingly, ifthe negative determination is made in step 20, the process proceeds tostep 26 so as to clear the malfunction determining opportunity counterECDEGOF, and then the process is returned to step 10.

On the other hand, if an affirmative determination is made in step 20,the process proceeds to step 22 where it is determined whether or notthe ECU 8 outputs a start signal so as to start operation of the EGR 1.As mentioned above, the malfunction determination is a process fordetermining whether or not the EGR valve 4 is in a fixed state where theEGR valve 4 is fixed to be in an open state. Thus, the malfunctiondetermining process must be performed under the condition in which theECU 8 controls the EGR valve 4 to close the valve.

Accordingly, if a negative determination is made in step 22, that is, ifthe ECU 8 controls the EGR valve 4 to open, the process proceeds to step26 so as to clear the malfunction determining opportunity counterECDEGOF since the malfunction determining process cannot be performed.The process is then returned to step 10.

On the other hand, if an affirmative determination is made in step 22,the process proceeds to step 24 where it is determined whether or notthe above-mentioned EGRON history flag XJEGON is set (XJEGON=ON). Asmentioned above, the EGRON history flag indicates an operational historyof the EGR valve 4 that the EGR valve 4 was operated at least one time.Additionally, as described with reference to FIGS. 5 and 6, if thedetermination of occurrence of malfunction is made before the EGR valveis operated, is it possible to make an erroneous determination and,thus, an accurate determination of occurrence of malfunction cannot beachieved.

Accordingly, if the negative determination is made in step 24, that is,if there is no history indicating an operation of the EGR valve 4, it ispossible that an accurate determination cannot be made. Thus, in such acase, the process proceeds to step 26 so as to clear the malfunctiondetermining opportunity counter ECDEGON, and then the process isreturned to step 10.

On the other hand, if an affirmative determination is made in step 24,the process proceeds to step 28 where it is determined whether or not apredetermined delay time t3 has elapsed. The delay time t3 is a timeperiod corresponding to a single step of the malfunction determiningopportunity counter ECDEGOF shown in FIG. 6-(B). That is, the passage ofdelay time t3 is waited in step 24, and then the process proceeds tostep 30.

In step 30, the malfunction determining process is performed. Themalfunction determining process performed in step 30 is shown in FIG. 4.When the malfunction determining process of step 30 is completed, theprocess proceeds to step 32 where the malfunction determiningopportunity counter ECDEGOF used in step 14 and the counter used in step16 are cleared, and then the process is ended.

As mentioned above, in the start condition determining process accordingto the present embodiment, the EGRON history flag XJEGON is provided, instep 12, to indicate the operation history of the EGR valve 4. Then, themalfunction determining process is performed, in step 30, only when itis determined, in step 24, that the operation history is present basedon the EGRON history flag XJEGON. Thus, an erroneous determination canbe prevented, and an accurate determination of occurrence of malfunctioncan be performed.

A description will now be given, with reference to mainly FIGS. 4 and 7,of the malfunction determining process of the EGR 1 performed in step30. As mentioned above, FIG. 4 is a flowchart of the malfunctiondetermining process, and FIG. 7 shows an example of an operation of theEGR 1 in which the malfunction determining process is performed. In FIG.7, (A) indicates a vehicle speed SPD which is obtained from the outputof the vehicle speed sensor 39. Additionally, (B) indicates a counter(hereinafter referred to as IDLON counter) which is provided in the ECU8 and is started when the engine is in an idle state an movement of thevehicle is stopped. Further, (C) indicates a target valve opening degreeETLIFTD which is set in response to a condition of the engine. Thetarget valve opening degree ETLIFTD is calculated by ECU 8.Additionally, (D) indicates a target valve opening degree blunted valueETLIFTDD which is a blunted value of the target valve opening degreeETLIFTD. Further, (E) indicates an actual degree (actual valve openingdegree) of opening of the EGR valve which is obtained from the output ofthe lift sensor 19.

The malfunction determining process shown in FIG. 4 is started after theEGR valve is operated and conditions for determining the start of themalfunction determination in steps 16 to 22 are satisfied, as isapparent from the description with reference to FIG. 3. When themalfunction determining process is started, it is determined, in step300, whether or not a normal determining history flag XNORMAL is set(XNORMAL=ON).

The normal determining history flag XNORMAL is a flag which is set whenit is determined, in the step 300 described later, that the operation ofthe EGR 1 is normal. Accordingly, it can be determined whether or not anormal determination, which indicates that the operation of the EGR 1 isnormal, was made in a previous malfunction determining process bychecking the status of the normal determining history flag XNORMAL. Itshould be noted that the reason for providing the step 300 will bedescribed later for the sake of convenience. If a negative determinationis made in step 300, the process proceeds to step 302. In step 302, itis determined whether or not the engine 2 is in an idle state and alsoif a time period during which the vehicle is stopped exceeds apredetermined time period t4. Similar to the above-mentioned step 18, itcan be determined by the output of the throttle switch 54 whether or notthe engine 2 is in an idle state. Additionally, it can be determined bythe output signal of the vehicle speed sensor 39 whether or not thevehicle is in a stop state.

In step 302, a wait occurs for the passage of the time period t4 whilethe engine is in an idle state and the vehicle is in a stopped state.This is because, immediately after the vehicle is stopped from themoving state and the engine assumes an idle state, it is possible thatthe EGR valve 4 is not fully opened even if the EGR 1 is normal.

That is, as mentioned above, the EGR valve 4 is operated by switchingthe negative pressure in the intake line and the atmospheric air in theVSV 5 which is controlled by the ECU 8. Thus, there is a delay in theoperation from the time when the ECU 8 outputs a signal for opening theEGR valve 4 to the time when the EGR valve 4 is actually opened.Accordingly, it is possible that if the determination of occurrence ofmalfunction is performed during the delay of operation, an erroneousdetermination is made. The time period t4 is set in response to thedelay in operation. Accordingly, in step 302, the determination ofoccurrence of malfunction is not performed immediately after the engineidle state and the vehicle is in a stopped state. The process after step304 is performed after waiting for the time period t4.

In the present embodiment, the passage of the time period t4 isdetermined by the IDLON counter which is indicated in FIG. 7-(B). Asmentioned above, the IDLON counter is a counter which counts when theengine is in an idle state and the vehicle is in a stopped state. Thus,the passage of the time period t4 can be determined based on the IDLONcounter. It should be noted that the time T1 in FIG. 7 indicates thetime when the time period t4 has elapsed.

If an affirmative determination is made in step 302, the processproceeds to step 304 so as to wait for a start of the vehicle. Then, theprocess proceeds to step 306 when the vehicle is started. It should benoted that the determination as to whether or not the vehicle is startedcan be made based on the output signal of the vehicle speed sensor 39.In the example of FIG. 7, the vehicle is started at the time T2.

In step 306, it is determined for the first time whether or not the EGR1 is operated. Specifically, in step 306, it is determined for the firsttime whether or not the EGR 1 is operated after the time period t4 iselapsed (step 302) during which the engine is in an idle state and thevehicle is in a stopped state. If the affirmative determination is madein step 306, the process after step 308 is performed which is anessential process for determining an occurrence of malfunction. Owing tothe process of step 306, an operation of the EGR 1 can be started underthe condition in which the operation of the engine is stable. It shouldbe noted that, in the example of FIG. 7, the EGR 1 is operated for firsttime at the time T3.

When an operation of the EGR 1 is started, the ECU 8 starts the EGRcontrol process. FIG. 11 is a flowchart showing the EGR control process.A description will now be given, with reference to FIG. 11, of the EGRcontrol process performed by the ECU 8. It should be noted that the EGRcontrol process is separately performed form the malfunction determiningprocess shown in FIG. 4.

When the EGR control process shown in FIG. 11 is started, the ECU reads,in step 400, the operating conditions of the engine 2 based on varioussensors. Then, in step 402, the ECU 8 calculates a degree of opening(target valve opening degree ETLIFTDD) of the EGR valve 4 which isoptimum for the operating conditions of the engine 2 that was read instep 400.

In step 404, a blunt process is performed by a known method with respectto the target valve opening degree ETLIFTD calculated in step 402 so asto obtain the target valve opening degree blunted value ETLIFTDD. Itshould be noted that step 404 is provided for performing the malfunctiondetermining process, and the detail will be described later.

In step 406, the operation of the SVS 5 is controlled based on thetarget valve opening degree ETLIFTD obtained in step 402 and the actualvalve opening degree ELIFTD of the EGR valve 4 which was calculated bythe output signal of the lift sensor 19. A feedback control is performedso that the actual valve opening degree ELIFTD of the EGR valve 4becomes equal to the target valve opening degree ETLIFTD. Thereby, theEGR valve 4 is controlled to be at an optimum degree of opening. Thus,an appropriate amount of exhaust gas is returned to the intake passage 9via the exhaust gas recirculating passage 3 so as to reduce NOx. In step408, an EGR start flag XAEGR is set which indicates that an operation ofthe EGR 1 is started (EGR ON) by the ECU 8, and then the process isended. It should be noted that the above-mentioned EGR process isrepeatedly performed for predetermined periods.

Returning now to FIG. 4, the description of the malfunction determiningprocess is continued.

If an affirmative determination is made in step 306, the processproceeds to step 308. In step 308, the target valve opening degreeETLIFTD, the target valve opening blunted value ETLIFTDD and the actualvalve opening degree ETLIFTD are read at the time T3 when the EGR isfirst operated after the time period t4 has passed during which theengine is in an idle state and the vehicle is in a stopped state. Thetarget valve opening degree ETLIFTD, the target valve opening bluntedvalue ETLIFTDD and the actual valve opening degree ETLIFTD are stored inthe ROM 41 as a reference target valve opening degree ETLIFTD1, areference target valve opening blunted value ETLIFTDD1 and a referenceactual valve opening degree ETLIFTD1, respectively.

The target valve opening degree ETLIFTD is calculated in step 402 of theEGR control process shown in FIG. 11. The target valve opening degreeblunted value ETLIFTDD is calculated in step 404 of the EGR controlprocess shown in FIG. 11. The actual valve opening degree ELIFTD isobtained from the output of the lift sensor 19. The reference targetvalve opening degree ETLIFTD1, the reference target valve openingblunted value ETLIFTDD1 and the reference actual valve opening degreeETLIFTD1 correspond to the target valve opening degree ETLIFTD, thetarget valve opening blunted value ETLIFTDD and the actual valve openingdegree ETLIFTD at the time T3, respectively, as shown in FIG. 7-(C), (D)and (E).

In step 310, it is determined whether or not a difference(ETLIFTD-ETLIFTD1) between the target valve opening degree ETLIFTD andthe reference target valve opening degree ETLIFTD11 is equal to orgreater than a predetermined value α. If an affirmative determination ismade in step 310, the process proceeds to step 312. It is determined, instep 312, whether or not the difference (ELIFTD-ELIFTD1) between theactual valve opening degree ELIFTD and the reference actual valveopening degree ELIFTD1 is equal to or greater than a predetermined valueβ. If it is determined, in step 312, that the difference(ELIFTD-ELIFTD1) is equal to or greater than the predetermined value β,a determination in made in step 314 that the EGR 1 is normal.

A description will now be given of the reason why it can be determinedthat the EGR 1 is normal when the above-mentioned determinations insteps 308 to 312 are made.

As mentioned above, the target valve opening degree ETLIFTD iscalculated by the ECU 8 in response to the operating conditions of thevehicle, and indicates the valve opening degree of the EGR valve 4 whichis optimum for the operating conditions. Accordingly, the ECU 8 controlsthe operation of the EGR valve 4 via the VSV 5 so that the actual valveopening degree ELIFTD becomes equal to the target valve opening degreeETLIFTD.

Supposing that a malfunction occurs in the VSV 5 or the EGR valve 4which constitutes the EGR 1, the EGR valve 4 does not operate when theECU 8 controls the EGR valve to operate. On the other hand, since thetarget valve opening degree ETLIFTD is calculated by the ECU 8, it canbe calculated even when a malfunction occurs in the EGR 1. Accordingly,when a malfunction occurs in the EGR 1, the target valve opening degreeETLIFTD varies in accordance with operating conditions whereas theactual valve opening degree ELIFTD of the EGR valve 4 does not vary.

Considering the above-mentioned phenomenon, in the present embodiment,it is determined that the EGR valve 4 is normal when the actual valveopening degree ELIFTD is changed in response to the change in the targetvalve opening degree by the predetermined valve α.

Additionally, in the present embodiment, the determination of occurrenceof malfunction is not based on a difference between the target valveopening degree and the actual valve opening degree as is in theconventional method but the determination is based on the difference(ELIFTD-ELIFTD1) between the actual valve opening degree ELIFTD and thereference actual valve opening degree ELIFTD1 when the target valveopening degree ETLIFTD is changed by a value greater than thepredetermined valve α from the reference target valve opening degreeETLIFTD1. That is, in the present embodiment, the determination ofoccurrence of malfunction is performed based on the absolute value(ELIFTD-ELIFTD1) of the change in the actual valve opening degree. Thereason for this is described below.

As mentioned above, since the target valve opening degree ETLIFTD iscalculated in accordance with operating conditions of the engine 2, thetarget valve opening degree continuously changes due to change in theoperating conditions of the engine 2. On the other hand, since the EGRvalve 4 is controlled by using the VSV 5, the actual valve openingdegree ELIFTD cannot follow or respond to the target valve openingdegree ETLIFTD without delay. Accordingly, it is possible that anaccurate determination of malfunctioning cannot be made in theconventional method in which the determination of malfunctioning is madebased on the actual valve opening degree ELIFTD and the target valveopening degree ETLIFTD since both the actual valve opening degree ELIFTDand the target valve opening degree ETLIFTD include a variation factor.

On the other hand, in the malfunction determining process according tothe present embodiment, the change in the target valve opening degreeETLIFTD does not directly influence the determination of malfunctioningsince target valve opening degree is used only for setting a timing forperforming the determination of a malfunction. Additionally, even if thedelay in response of the operation of the EGR valve 4 is generated, thedelay is canceled when the difference (ELIFTD-ELIFTD1) is calculatedsince the determination of a malfunction is performed based on theabsolute value (the difference: ELIFTD-ELIFTD1) of the actual valveopening degree ELIFTD which is obtained from the reference actual valveopening degree ELIFTD1. Thus, in the malfunction determining processaccording to the present embodiment, the influence of the delay inresponse of the EGR valve 4 to the malfunction determining process canbe prevented, and thus an accurate determination of malfunctioning canbe made.

Returning now to FIG. 4 to continue the description of the process. Whenthe determination that the EGR 1 is normal is made in step 314, theprocess proceeds to step 316 where the normal determination history flagXNORMAL is set (XNORMAL=ON). That is, the normal determination historyflag XNORMAL is set only when the EGR 1 is determined to be normal instep 314.

A reference is now made to the above-mentioned step 300. In step 300,the process is ended without performing the malfunction determiningprocess after step 302 when the normal determination history flagXNORMAL is set. That is, once the normal determination was made in step300, the execution of the process after step 302 is prohibited until thenext time the engine 2 is started. The reason for this is describedbelow.

The malfunction determining process is performed based on variousparameters such as the above-mentioned target valve opening degreeETLIFTD, the target valve opening degree blunted value ETLIFTDD, theactual valve opening degree ELIFTD, the reference target valve openingdegree ETLIFTD1, the reference target valve opening degree blunted valueETLIFTDD1 and the reference actual valve opening degree ELIFTD1. Each ofthe parameters is calculated based on the output signals output from thelift sensor 19 and various sensors which detects operating conditions ofthe engine 2. It is possible that external noise can intrude into eachof these sensors. If the external noise intrudes and an erroneous signalis generated, an accurate determination may not be performed.

Accordingly, in the structure in which the malfunction determiningprocess is continued after once the normal determination was made, it ispossible that the EGR control process cannot be appropriately performedwhen the above-mentioned intrusion of external noise occurs since themalfunction determining process is performed despite the EGR 1 beingnormal. Thus, in the present embodiment, when the normal determinationthat the EGR 1 is normal is made in step 314, the normal history flagXNORMAL is set (XNORMAL=ON) in step 316, whereas the malfunctiondetermining process after step 302 is not performed when the normaldetermination flag XNORMAL is set in the process of step 300.Accordingly, accuracy of the malfunction determining process can beincreased. It should be noted that the malfunction determining processis ended when the process of step 316 is completed.

The above description is for the process when it is determined, in step312, that the difference (ELIFTD-ELIFTD1) between the actual valveopening degree ELIFTD and the reference actual valve opening degreeELIFTD1 is equal to or greater than the predetermined value β, that is,when it is determined that the EGR 1 is normal. On the other hand, if anegative determination is made in step 312, the process proceeds to step318.

In step 318, it is determined whether or not the EGR 1 is continuouslyoperated for a predetermined time period t5. The process of step 318 isprovided to let the delay time pass so as to prevent influence of thedelay in response of the EGR valve 4 when the malfunction determiningprocess is performed, in step 318, based on the target valve openingdegree blunted value ETLIFTDD. If the negative determination is made instep 318, the process returns to step 310.

On the other hand, if an affirmative determination is made in step 318,the process proceeds to step 320. In step 320, it is determined whetheror not the difference (ETLIFTDD-ETLIFTDD1) between the target valveopening blunted value ETLIFTDD and the reference target valve openingdegree ETLIFTDD1 is equal to or greater than a predetermined value τ. Ifan affirmative determination is made in step 320, the process proceedsto step 322. On the other hand, if a negative determination is made, theprocess returns to step 310.

In step 322, the malfunction determining counter CDEGOF is incremented.Then, in step 324, it is determined whether or not the malfunctiondetermining counter CDEGOF is equal to or greater than a predeterminedvalue X. If it is determined, in step 324 that the malfunctiondetermining counter is equal to or greater that the predetermined valueX (CDGOE≧X), the process proceeds to step 326 to make a determinationthat the EGR 1 is malfunctioning.

It should be noted that, steps 322 and 324 are provided, and thedetermination of malfunctioning in step 324 is not made immediatelyafter the determination of malfunctioning is made in steps 310 to 312.The determination of malfunctioning in step 24 is made when themalfunction determining counter CDEGOF becomes equal to or greater thanthe predetermined value X. The reason for this is to prevent adetermination of malfunctioning when an erroneous determination is madedue to intrusion of external noise.

As apparent from the above description, in the malfunction determiningprocess according to the present invention, the determination ofmalfunctioning is not made immediately after the processes of steps 310and 312 resulted in the difference (ETLIFTD-ETLIFTD1) between the targetvalve opening degree ETLIFTD and the reference target valve openingdegree ETLIFTDD being equal to or greater than the predetermined value αand the difference (ELIFTD-ELIFTD1) between the actual valve openingdegree ELIFTD and the reference valve opening degree ELIFTD1 is equal toor greater than the predetermined value β. That is, the process formalfunctioning in steps 322 to 326 is initiated when it is determined,in step 320, that the difference (ETLIFTDD-ETLIFTDD1) between the targetvalve opening degree blunted value ETLIFTDD and the reference targetvalve opening degree ETLIFTDD1 is equal to or greater than thepredetermined value τ.

In the malfunction determining process according to the presentembodiment, the process for determining occurrence of malfunction isperformed when the changes in the target valve opening degree and thetarget valve opening degree blunted value are equal to or greater thanthe predetermined values α and τ, respectively, and when the change inthe actual valve opening degree is less than the predetermined value β.

A description will now be given, with reference to FIGS. 8 and 9, of thereason for the above-mentioned procedure. FIGS. 8 and 9 areillustrations in which the target valve opening value ETLIFTD, thetarget valve opening degree blunted value ETLIFTDD and the actual valveopening value ELIFTD are indicated in an overlapping relationship forthe sake of convenience. Additionally, the reference target valveopening degree ETLIFTD1, the reference target valve opening bluntedvalue ETLIFTDD1 and the reference actual valve opening degree ELIFTD1are indicated in the figures as they are the same value. Further, Thepredetermined value α in step 310 and the predetermined value τ in step320 are indicated by a dashed line indicated by an arrow A (in exampleof the figure, a=τ). The predetermined value β in step 312 is indicatedby a solid line indicated by an arrow B.

As mentioned above, since the target valve opening value ETLIFTD iscalculated by the ECU 8, the ECU 8 sets the target valve opening valueETLIFTD immediately after it is calculated. A time T5 shown in FIG. 8indicates the time when the ECU 8 sets the target valve opening degreeETLIFTD. Since the target valve opening degree ETLIFTD is set by anelectrical process by the ECU 8, the target valve opening degree ETLIFTDhas a characteristic in that a sharp increase occurs at the time T5.Thus, for the sake of simplification in FIG. 8, the change in the targetvalve opening degree ETLIFTD is indicated by a square form.

Additionally, in the example in the figures, it is assumed thatdifference (ETLIFTD-ETLIFTD1) between the target valve opening degreeETLIFTD and the reference target valve opening degree ETLIFTD1 is equalto or greater than the predetermined value α. That is, the condition isachieved where the affirmative determination is made in step 310.

On the other hand, referring to the actual valve opening degree ETLIFTD,since there is the delay of response in an operation of EGR valve 4 asmentioned above, the change in the actual valve opening degree ELIFTD isdelayed with respect to the change in the target valve opening degreeETLIFTD.

Thus, in a structure in which the malfunction determining process isperformed based on the amount of change in the actual valve openingdegree (difference: ELIFTD-ELIFTD1) when the amount of change(difference: ETLIFTD-ETLIFTD1) in the target valve opening degreeETLIFTD is equal to or greater than the predetermined value α, that is,when the determination of malfunctioning is performed only by theprocess of steps 310 and 312, it is possible that the amount of change(difference: ELIFTD-ELIFTD1) in the actual valve opening degree ELIFTDis less than the predetermined value β during the time period t5 shownin FIG. 8 despite the EGR 1 being normal. Thus, if a determination isperformed during the time period t5, it is possible to make an erroneousdetermination.

On the other hand, since the target valve opening degree blunted valueETLIFTDD is a blunted or filtered value of the target valve openingdegree ETLIFTD, the target valve opening degree blunted value ETLIFTDDis closer to the actual valve opening degree ELIFTD than the targetvalve opening degree ETLIFTD. Thus, an occurrence of an erroneousdetermination due to the delay in response mentioned above can beprevented by performing the determination of malfunctioning when theamount of change (difference: ETLIFTDD-ETLIFTDD1) in the target valveopening degree blunted value is equal to or greater than thepredetermined value τ.

On the other hand, if the time for initiating the malfunctiondetermining process is set based on the target valve opening degreeETLIFTDD alone, the following problem may be raised. If the target valveopening degree ETLIFTD sharply changes at a time T6 from a certain value(for example, zero) and then returns to the certain value at a time T7as shown in FIG. 9, such a change cannot be reflected to the targetvalve opening degree blunted value ETLIFTDD. Thus, it is possible thatthe target valve opening degree blunted value may be maintained at agreater value than the predetermined value τ for the time period t6.

Additionally, since the EGR valve 4 is controlled so that the actualvalve opening degree ELIFTD becomes equal to the target valve openingdegree ETLIFTD, it is possible that the actual valve opening degreeELIFTD becomes equal to zero (closed state) during the time period (fromtime T6 to time T7) when the target valve opening degree ELIFTD is zero.If the time for initiating the malfunction determining process is setbased on the target valve opening degree blunted value ETLIFTDD alone,the determination of malfunctioning is performed under theabove-mentioned condition. Accordingly, an erroneous determination maybe made that the EGR 1 is malfunctioning since the amount of change(difference: ELIFTD-ELIFTD1) in the actual valve opening degree ELIFTDis regarded as less than the predetermined value β despite the actualvalve opening degree ELIFTD being zero due to a normal operation of theEGR valve 4.

Accordingly, in the present embodiment, steps 310 and 320 are providedso that the malfunction determining process after step 322 is performedwhen the two conditions are established that the amount of change(difference: ETLIFTD-ETLIFTD1) in the target valve opening degreeETLIFTD is equal to or greater than the predetermined value α and theamount of change (difference:ETLIFTDD-ETLIFTDD1) is equal to or greaterthan the predetermined value τ, and further when it is determined, instep 312, that the difference (ELIFTD-ELIFTD1) is less than thepredetermined value β. From this procedure, the above-mentionederroneous determination is prevented, and thus an accurate malfunctiondetermining process can be performed.

In the above-mentioned present embodiment, when the determination thatthe apparatus is normal is made in the process in steps 310 and 312, themalfunction determining process using the target valve opening degreeblunted value in step 320 is not performed. That is, when thedetermination of normal is made in the process of steps 310 and 312, thedetermination of normal is made irrespective of the change in the targetvalve opening degree blunted value ETLIFTDD. The reason for this isdescribed below with reference to FIG. 10.

It is assumed that the engine 2 is operated in a a high load condition.As mentioned above, since the EGR valve 4 is operated by the intakenegative pressure in the intake passage 9, if the throttle valve 11 isfully opened due to the high load operation, the intake negativepressure is decreased which leads to a state where the intake pressureis equal to the atmospheric pressure.

In such a condition, the EGR valve 4, which is driven by the intakenegative pressure, cannot be operated properly. Thus, the referenceactual valve opening degree shifts toward the fully closed state asshown in FIG. 10. However, since the target valve opening value ETLIFTDand the target valve opening degree blunted value ETLIFTDD are set bythe calculation of the ECU 8, it is possible that the amount of changesin the target valve opening degree ETLIFTD and the target valve openingdegree blunted value ETLIFTDD are greater than the predetermined valueswhen the EGR valve 4 is in the closed state due to the operatingconditions of the engine 2.

Accordingly, if the malfunction determining process is performed duringa period (a period indicated by t7 in FIG. 10) in which the referenceactual valve opening degree ELIFTD1 of the EGR valve 4 is zero due tooperating conditions of the engine 2, it is possible that an erroneousdetermination is made that the EGR 1 is malfunctioning as the amount ofchange (difference:ELIFTD-ELIFTD1) in the actual valve opening degreeELIFTD is less than the predetermined value β despite the EGR 1performing a normal operation.

However, even if the apparatus is operated in the above-mentionedcondition, the EGR valve 4 always operates to some degree when anoperation of the EGR 1 is initiated and the target valve opening degreeETLIFTD is established. This operation can be detected by the liftsensor 19. That is, even if the engine is in a high-load condition, itcan be determined that the EGR 1 is normal if the amount of change inthe actual valve opening degree ELIFTD is greater than a predeterminedvalue when the amount of change in the target valve opening degree isgreater than a predetermined value,.

On the other hand, since the target valve opening degree blunted valueETLIFTDD is raised with a delay with respect to the target valve openingdegree ETLIFTDD, if the initiation of the malfunction determiningprocess is delayed till the amount of change (ETLIFTDD-ETLIFTDD1) hasbecome equal to or greater than the predetermined value τ, it ispossible that the engine 2 goes into a high-loaded condition during thatperiod.

Accordingly, in this embodiment, the target valve opening degree bluntedvalue ETLIFTDD is not used as a parameter for determination of a normaloperation. The EGR 1 is determined to be normal if the amount of change(difference: ELIFTD-ELIFTD1) of the actual valve opening degree ELIFTDis equal to or greater than the predetermined value β when the amount ofchange (ETLIFTD-ETLLIFTD1) of the target valve opening degree ETLIFTD isequal to or greater than the predetermined value α in the process ofstep 310 and 312.

It should be noted that, in the above-mentioned embodiment, steps 10, 12and 24 in FIG. 3 correspond to means for determining whether the exhaustgas recirculating valve is operated at least once. Additionally, steps310, 312 and 320 in FIG. 4 correspond to means for determiningmalfunction.

Additionally, in the above-mentioned embodiment, the amount of changewith respect to the actual valve opening degree, the target valveopening degree and the target valve opening degree blunted value isobtained as the "difference" from the reference values, the amount ofchange may be represented by a "ratio" to perform a similar process.

Further, in the above-mentioned embodiment, means for detecting theoperating conditions of the valve is not limited to a method in whichthe valve opening degree is directly detected. For example, atemperature in the EGR passage can be detected and the operatingcondition of the valve can be detected by a temperature in the EGRpassage.

The present invention is not limited to the specifically disclosedembodiments, and variations and modifications may be made withoutdeparting from the scope of the present invention.

What is claimed is:
 1. A malfunction determining apparatus of an exhaustgas recirculation system having an exhaust gas recirculating valveprovided between an exhaust passage and an intake passage of an internalcombustion engine of a vehicle, said malfunction determining apparatuscomprising:means for detecting an actual degree of opening of saidexhaust gas recirculating valve; means for determining whether saidexhaust gas recirculating valve has been operated at least once; andmeans for determining whether said exhaust gas recirculation system ismalfunctioning based on the actual degree of opening of said exhaust gasrecirculating valve after said exhaust gas recirculating valve has beenoperated at least once.
 2. The malfunction determining apparatus asclaimed in claim 1, further comprising start condition determining meansfor determining an appropriate condition to start a malfunctiondetermining process to make said determination of malfunctioning basedon operating conditions of said internal combustion engine and saidvehicle.
 3. The malfunction determining apparatus as claimed in claim 2,wherein said start condition determining means determines whether saidinternal combustion engine has been operated for a predetermined periodso as to start said malfunction determining process after said internalcombustion engine has been operated for said predetermined period. 4.The malfunction determining apparatus as claimed in claim 2, whereinsaid start condition determining means determines whether said internalcombustion engine is in an idle operation and said vehicle is a stoppedstate so as to start said malfunction determining process when saidinternal combustion engine is in an idle operation and said vehicle isin said stopped state.
 5. The malfunction determining apparatus asclaimed in claim 2, wherein said start condition determining meansdetermines whether a cooling water temperature of said internalcombustion engine is one of equal to and greater than a predeterminedtemperature so as to start said malfunction determining process when thecooling water temperature of said internal combustion engine is one ofequal to and greater than said predetermined temperature.
 6. Themalfunction determining apparatus as claimed in claim 2, wherein saidstart condition determining means determines whether said exhaust gasrecirculating valve is controlled to be in a closed state so as to startsaid malfunction determining process when said exhaust gas recirculatingvalve is controlled to be in said closed state.